1. Field of the Invention
The present invention relates generally to chromatic dispersion compensation control methods and apparatuses, and more particularly to a chromatic dispersion compensation control method and apparatus that compensate for the chromatic dispersion of an optical transmission path using a tunable dispersion compensator.
2. Description of the Related Art
In a long distance optical transmission system, the waveforms of light signal pulses are degraded or deformed by transmission due to the chromatic dispersion of an optical fiber that is a transmission path.
FIG. 1 is a diagram for illustrating intersymbol interference according to bit rates.
Referring to FIG. 1, the effect of waveform degradation is limited in a low bit rate (for example, 2.5 Gbps) optical transmission system. However, in a high bit rate (for example, 10 Gbps) optical transmission system where the pulse width of data is reduced, the effect of intersymbol interference is conspicuous due to waveform degradation, thus causing the degradation of error characteristics.
FIG. 2 is a diagram for illustrating a conventional chromatic dispersion compensation control method.
In the existing 10G optical transmission system, it is predominant to insert a dispersion compensation fiber opposite in characteristics to an optical fiber transmission path 1 as a dispersion compensator 2 in the transmission path 1 between a transmitter 4 and a receiver 3 in order to correct waveform degradation. However, it is necessary to prepare dispersion compensation fibers having different lengths as the dispersion compensator 2 depending on the length (distance) of the transmission path 1. Accordingly, tunable dispersion modules have also been used recently as the dispersion compensator 2 in place of dispersion compensation fibers.
The amount of control of the dispersion compensator 2 is set to an optimum value depending on the type of transmission path fiber, such as SMF (Single Mode Fiber), NZDSF (Non-Zero Dispersion Shifted Fiber), or DSF (Dispersion Shifted Fiber), and its transmission distance. In response to subsequent changes over time in the fiber characteristics or subsequent replacement of the transmission path fiber, the dispersion compensator 2 should be adjusted during its operation in order to maintain transmission quality.
FIG. 3 is a block diagram showing a dispersion control method of a conventional tunable dispersion compensator. FIG. 4 is a flowchart showing the dispersion control method of the conventional tunable dispersion compensator.
In the case of using a tunable dispersion compensator as the dispersion compensator 2, according to the dispersion control method of the conventional tunable dispersion compensator, the dispersion compensation is varied in amount in each of the direction to decrease residual dispersion and the direction to increase residual dispersion while monitoring the amount of error in the receiver 3, and the direction in which to control the dispersion compensation is determined based on whether the amount of error tends to increase or decrease as shown in the block diagram of FIG. 3 and the flowchart of FIG. 4.
Japanese Laid-Open Patent Application No. 2004-304559 shows determining whether the dispersion compensation is in the positive or negative direction by comparing the peak value of a received signal with a predetermined threshold.
According to the conventional method shown in FIGS. 3 and 4, the dispersion compensation is varied in each of the direction to decrease residual dispersion and the direction to increase residual dispersion. Therefore, as shown in FIG. 5, in the case of varying the dispersion compensation in the direction to increase residual dispersion from a residual dispersion A, where the error rate is close to an error correctable level threshold (upper limit) TH, there is a problem in that the error rate may exceed the error correctable level threshold TH so that it may not be possible to correct errors so as to temporarily increase errors.